Document using managed identity with New-AzSqlDatabaseExport

[matthetherington]

Description

Update New-AzSqlDatabaseExport documentation to cover using managed identity for authentication with SQL server and the target storage account.

Mandatory Checklist

• Please choose the target release of Azure PowerShell. (⚠️Target release is a different concept from API readiness. Please click below links for details.)

  • General release
  • Public preview
  • Private preview
  • Engineering build
  • No need for a release

• Check this box to confirm: I have read the Submitting Changes section of CONTRIBUTING.md and reviewed the following information:

• SHOULD update ChangeLog.md file(s) appropriately
  • Update src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md.
    • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header in the past tense.
  • Should not change ChangeLog.md if no new release is required, such as fixing test case only.
• SHOULD regenerate markdown help files if there is cmdlet API change. Instruction
• SHOULD have proper test coverage for changes in pull request.
• SHOULD NOT adjust version of module manually in pull request

[azure-client-tools-bot-prd]

Thanks for your contribution! The pull request validation has started. Please revisit this comment for updated status.

[microsoft-github-policy-service]

Thank you for your contribution @matthetherington! We will review the pull request and get back to you soon.

[matthetherington]

@microsoft-github-policy-service agree

[matthetherington]

Remade this under #27881 after receiving a message about my fork containing sensitive info that was removed from the main repo